Phys649: Physics of Optoelectronic Devices

Phys649: Physics of Optoelectronic Devices Alexey Belyanin Room 426 MIST Office: 845-7785 Cell: 324-3071 Email: belyanin@tamu.edu Office hours: any time when I am in the office

Textbook: E. Rosencher, B. Vinter, Optoelectronics, 2002 • Concise presentation of main sub-disciplines • Emphasis on the key unifying concepts • Utilitarian approach: useful technique as opposed to • hard-nosed theory • We won’t cover all chapters (e.g. quantum • mechanics) but you can use them as a reference

Policy • Attendance is required (20% of the grade) • Questions, discussion are encouraged; • I may adjust the contents of the course as we go • Homework assignments every week (50%) • Short presentations in the end of the semester on a topic of your choice (30%)

Optoelectronics: • Studies electronic devices that interact with light • Interact = emit, absorb (detect), modulate, switch • Light = any EM radiation • “Electronic” usually means “made of semiconductors”. Emerging technologies: plasmonics, graphene, carbon nanotubes, molecular devices, … Interplay between quantum electronics, nonlinear optics, physics of semiconductors, and transport phenomena It is important not to get lost and to see coherence and connections between different subjects

Main devices: - semiconductor lasers and detectors, - nonlinear optical systems - novel devices (carbon-based, plasmonic) Plan of study for each kind of devices: • Basic principles and device physics • Examples of state of the art devices • Challenges and outlook for the future Integrated photonics, nanodevices, quantum optical systems (cryptography, communications, …) Always with applications in mind, but emphasis on physics perspective

Physics laid down foundation for the Information Age • Stone Age • Bronze Age • Iron Age • Ice Age What is Information Age?

Information Age The cost of the transmission, storage and processing of data has been decreasing extremely fast Information is available anytime, any place, and for everyone Information and knowledge became a capital asset All of this became possible because of several revolutionary ideas

Transistor Laser and semiconductor laser Computer World-Wide Web Optical fibers Integrated circuits … Are invented by physicists

History of the WWW

History of the WWW • First proposal: Tim Berners-Lee (CERN) • in 1989 • 1991: First WWW system released by CERN • to physics community; first Web server in the US (SLAC) • 1993: University of Illinois releases user-friendly • Mozaic server • Currently: WWW is one of the most popular Internet • applications; >100 million users in the US alone

Invention of Computer • The first digital electronic computer was invented by • Theoretical Physics Prof. John Vincent Atanasoff • in 1937. It was built by Atanasoff and his graduate • student Clifford Berry at Iowa State College • in 1939 ($650 research grant). Basement of the Physics Dept. building where the Atanasoff-Berry Computer (ABC) was built.

ABC • Used base-two numbers (the binary system) - all • other experimental systems at the time used base-ten • Used electricity and electronics as it's principal media • Used condensers for memory and used a regenerative • process to avoid lapses that could occur from • leakage of power • Computed by direct logical action rather than by the • enumeration methods used in analog calculators Implemented principles of modern computers Only material base has been changed.

ABC Replica Berry with the ABC The drum – the only surviving fragment of ABC. It holds 30 numbers of 50 bits each. They are operated on in parallel. It is the first use of the idea we now call "DRAM" -- use of capacitors to store 0s and 1s, refreshing their state periodically. Card punch and reader

From ABC to ENIAC • 1940s: J. Mauchly and J. Eckert build ENIAC • (Electronic Numerical Integrator And Computer). All • basic concepts and principles of ENIAC are • “borrowed” from Atanasoff’s papers. • 1972: U.S. Court voids the Honeywell’s patent on the • computing principles and ENIAC, saying that it had • been “derived” from Atanasoff’s invention. • 1990: Atanasoff receives the U.S. National Medal of • Technology. He dies in 1995 at the age of 91.

Computers in the future may weigh no more than 1.5 tons. (Popular Mechanics, 1949) From ENIAC to … 1940's - IBM Chairman Thomas Watson predicts that "there is a world market for maybe five computers". 1950's - There are 10 computers in the U.S. in 1951. The first commercial magnetic hard-disk drive and the first microchip are introduced. Transistors are first used in radios. 1960's-70's - K. Olson, president, chairman and founder of DEC, maintains that "there is no reason why anyone would want a computer in their home." The first microprocessor, 'floppy' disks, and personal computers are all introduced. Integrated circuits are used in watches. ENIAC (1946) weighed 30 tons, occupied 1800 square feet and had 19,000 vacuum tubes.It could make 5000 additions per second

Impact of semiconductor electronics Before diodes and transistors: vacuum tubes • 1954-1963: SAGE Air Defense Project • 23 32-bit computers • Each contains 55,000 vacuum tubes, weighs • 250 tons, and consumes 3 Megawatt • Tracks 300 flights • Total cost: $60 billion (double the price of • Manhattan Project!) • Performance equivalent to $8 calculator • built on transistors!

Smaller, Denser, Cheaper Moore’s Law (1965): every 1.5 years the number of transistors on a chip is doubled Transistor of single-atom size by 2020?

Limit on the transistor size • Limit on the manufacturing • technology

Pushing Fundamental Limits:Challenges and Bottlenecks • Semiconductors: how small the device can be? • Memory and data storage: limits on writing density? • Communications: limits on data rate?

Telecommunications

Shannon-Nyquist Theorem In a communication channel with bandwidth B, the data rate (number of bits per second) can never exceed 2B Number of channels = Total bandwidth of the medium/B

How it all started … Samuel Morse's telegraph key, 1844. Today's information age began with the telegraph. It was the first instrument to transform information into electrical signal and transmit it reliably over long distances. Alexander Graham Bell’s commercial telephone from 1877. In 1880 patented a “Photophone” (air-based optical telephone)

Alexander Bell, Helen Keller, and Anne Sullivan, 1894

Speaking into the handset's transmitter or microphone makes its diaphragm vibrate. This varies the electric current, causing the receiver's diaphragm to vibrate. This duplicates the original sound. • Telephone connection requires a dedicated wire line; • Only one communication is possible at a time

Radio: communication through radio waves 1895 www.nrao.edu Frequency measured in Hertz 1 Hz = 1 cycle/second 1 kHz = 1000 cycles/second Alexander Popov Guglielmo Marconi How many channels are possible? How many signals can be transmitted at the same time??

Radio stations have to broadcast at different carrier frequencies to avoid cross-talk Human ear: 10 Hz-20 kHz Range of frequencies (Bandwidth) needs to be at least 20 kHz for each station Frequencies of different stations should be at least 20 kHz apart

About 100 bands from 0 to 2000 kHz Even if you transmit only voice, from 0 to 2000 kHz you can squeeze only 2000 kHz/20 kHz = 100 different “talks”. What if you want to download data?

Digital transmission: any signal, but transmission speed is still limited by bandwidth! Binary code is transmitted: “0s” and “1s” – bits of information Mega = million, Giga = billion; 1GHz = 1000 MHz = 109 Hz Want download speed of 2 Mb/sec? Need bandwidth at least 1 MHz Want 100 Mb/sec? need 50 MHz bandwidth just for yourself Modern cell phones and GPS use gigahertz (GHz) frequencies But this is only 1000 MHz/50 MHz = 20 channels at 100 Mb/sec!

Faster, faster, faster Higher carrier frequencies Wider bandwidth Higher data rate Using optical frequencies?! 100 THz !

What kind of medium can carry optical frequencies? Air? Only within line of sight; High absorption and scattering Optical waveguides are necessary! Copper coaxial cable? High absorption, narrow bandwidth 300 MHz Glass? Window glass absorbs 90% of light after 1 m. Only 1% transmission after 2 meters. Extra-purity silica glass?!

Loss in silica glasses What is dB? Increase by 3 dB corresponds to doubling of power Loss per km, dB Maximum tolerable loss Wavelength, nm Transmisson 95.5% of power after 1 km P = P(0) (0.995)N after N km P = 0.01 P(0) after 100 km: need amplifiers and repeaters Total bandwidth ~ 100 THz!!

How to confine light with transparent material?? n > n’ Total internal reflection!

Dielectric waveguides n > n’ Optical fiber! 1970: Corning Corp. and Bell Labs

Optical fibers Made by drawing molten glass from a crucible 1965: Kao and Hockham proposed fibers for broadband communication 1970s: commercial methods of producing low-loss fibers by Corning and AT&T. 1990: single-mode fiber, capacity 622 Mbit/s Now: capacity ~ 1Tbit/s, data rate 10 Gbit/s

Fibers open the flood gate Bandwidth 100 THz would allow 100 million channels with 2Mbits/sec download speed! Each person in the U.S. could have his own carrier frequency, e.g., 185,674,991,235,657 Hz. However, we are using less than 1% of available bandwidth! And maximum transmission speed is less than 0.00001 of bandwidth

Limitations of fiber communications In optical communications, information is transmitted as light signal along optical fibers However, if we want to modify, add/drop, split, or amplify signal, it needs to be first converted to electric current, and then converted back to photons This is a slow process (maximum 10 GHz)

All-optical switches Micro-Electro-Mechanical Systems (MEMS) 256 micro-mirrors (Lucent 2000)

THE DREAM: could we replace electrons with photons, and electric circuits with all-optical circuits? IBM website Futuristic silicon chip with monolithically integrated photonic and electronic circuits wires waveguides

Possible solutions: photonic crystals, plasmons, etc. Note T-intersections and tight bends, as in electric wires. You cannot achieve it in usual waveguides!

Digital Light Processing (DLP) Texas Instruments 1987 800x600, 1024x768, 1280x720, and 1920x1080 (HDTV) matrices of microscopic mirrors One mirror is only 16 microns! Less than 1/5 of human hair width http://www.dlp.com/includes/demo_flash.aspx

Laser mini-projectors Need microscopic lasers of all colors: red, blue, and green Fiber communications need mini lasers emitting in the near-infrared Smallest lasers are semiconductor lasers! mini laser projector

Excellent near infrared and red lasers Bad blue lasers Very bad green lasers

Problem of Interconnects Number of transistors grows, but this does not improve the performance as much. Reason? We use 21-century semiconductor devices and 19-century copper wires connecting them!

Electronic circuits: 45 nm wires, 1 million transistors per mm2 Computing speed is limited by inertia of electrons

The interconnect bottleneck • 109 devices per chip • Closely spaced metal wires slow down computation • Huge heat generated due to electric resistance

Can electronic circuits be replaced by photonic ones?! Using photons as bits of information instead of electrons would speed up the computing Photons travel much faster and don’t dissipate as much power However, large size of a photon would make computers 1000 times larger! Optical computer is still a dream. Even wilder dream is a quantum computer.

Quantum computer Bit Qubit (quantum bit): state of a single electron or atom Classical computer: 0 or 1 Quantum computer: 0 or 1 or any mixture of 0 and 1 Quantum computer can perform millions of calculations in parallel. Especially good for searching through huge amount of data or encoding/decoding secret messages Extremely fragile, needs low temperature

Conclusions • Microelectronics is approaching its fundamental limits. • Revolutionary ideas are needed! • - Single-molecule transistors? • - new materials, new fabrication technology? • Communication: how to increase speed? • - Novel lasers? • - All-optical network? • - How to make photons smaller? • New principles of computing??

Phys649: Physics of Optoelectronic Devices